South Korea and the United States kicked off a major combined military exercise Monday to reinforce deterrence against North Korea's nuclear and missile threats amid concern Pyongyang could use the maneuvers as a pretext for provocations.

The annual Freedom Shield exercise got under way for an 11-day run amid heightened tensions over Pyongyang's continued saber-rattling, including artillery firings near the western sea border and a series of missile launches.

The springtime exercise marks the first one after Pyongyang in November scrapped a 2018 inter-Korean military accord designed to reduce tensions along the border, raising concerns of the North possibly staging provocative military demonstrations.

Pyongyang has long denounced the allies' military drills as rehearsals for an invasion against it and has a track record of conducting missile launches in protest of such exercises, though Seoul and Washington have said such exercises are purely defensive.

The South Korean and US militaries said the latest exercise is aimed at improving their combined defense posture, noting that it will focus on multi-domain operations by utilizing land, sea, air, cyber and space assets, and countering the North's nuclear operations.

In a briefing last week, Col. Lee Sung-jun, the Joint Chiefs of Staff's spokesperson, said the exercise will simulate various scenarios and will include training on detecting and intercepting the North's cruise missiles.

The two sides plan to stage a total of 48 on-field drills this month -- more than double the number over a similar period last year -- although none of them are scheduled near the inter-Korean border, according to the South's military.

Personnel from 12 member states of the United Nations Command, including Australia, Britain, the Philippines and Thailand, will join the exercise, with the Neutral Nations Supervisory Commission observing them.

Members of the US-led multinational command, established in 1950 under a UN mandate to support South Korea during the 1950-53 Korean War, have regularly participated in the combined exercise.

The NNSC is tasked with overseeing the implementation of the armistice of the Korean War, which technically never ended as the warring sides did not sign a peace treaty.

Amid concerns of provocative acts by North Korea, South Korean and US reconnaissance aircraft were spotted flying over South Korea in apparent missions to monitor the North, according to flight trackers.

A US RC-135V plane, which took off from Okinawa, Japan, and a South Korean RQ-4 Global Hawk unmanned aircraft were seen flying over waters off the western coast and Gyeonggi Province, which surrounds Seoul. (Yonhap)